1. Field of the Invention
The present invention pertains to a multiple position tap changing switch including a plurality of rod type switch contacts and dual spaced apart roller type contactors for interconnecting the rod contacts and wherein the current flow paths through the contacts substantially balance opposing magnetic forces acting on the movable contactors.
2. Background
There are many applications of electrical transformers wherein a selected range of secondary voltages must be provided for the particular load associated with the transformer. One particularly severe duty application is in connection with transformers used for downhole deepwell electric motor driven pumps. Transformers used for these applications and similar severe duty uses must be provided with tap changing equipment wherein the secondary voltages can be varied to compensate for conductor cable voltage drop between the transformer and the pump, depending on the depth of the well, and changes in the density or viscosity of the fluid being pumped, for example, in applications for pumping crude oil from subterranean deposits. The wide voltage range normally required for transformer applications of the type described above as well as the fact that these transformers are usually situated in remote locations and are subject to frequent changes in secondary or output voltages has placed severe demands on the design of the tap changing switch used to select the voltage applied to the pump motor or other load.
The applications for transformers of the type described above dictate the use of a tap changing or selecting switch which is of the type which ruptures the current directly since the rated currents are usually moderate and mechanical simplicity and economy of manufacture are important considerations in the transformer system. However, in applications such as the deepwell pump application described herein the transformer is serving an induction motor which is started across the line and the transformer secondary circuit can experience current surges in the range of nine to ten times normal current flow. Accordingly, the switch structure must be rugged, capable of reliable snap action movement of the movable contactor between tap positions and offer minimum resistance to the operator in changing the tap position to prevent erratic or unwanted momentary connect and disconnect operations. In tis regard it is desirable to provide a switch which minimizes the force required to change tap positions, is mechanically stable in a selected position but is capable of rapid movement from one position to another to minimize arcing and interruption of power to the driven load. Accordingly, it is important to minimize unwanted forces acting on the movable contactors such as magnetic forces caused by current flow through the contact elements. Moreover, a smooth contact movement between tap positions together with a positive "snap-over" action is important.
It is also important to provide a tap changing switch which is positively lockable in a selected position and is difficult or impossible to inadvertently move between tap positions. Other considerations which must be dealt with include providing the switch structure with sufficient dielectric strength to prevent arcing or short circuiting and to permit access to the contact terminals for connecting and disconnecting the transformer winding leads from the transformer. All of these desiderata are difficult to satisfy with a switch design which is economical to manufacture and reliable in operation. However, the present invention provides a switch which meets the requirements which have been identified in the art but which have heretofore not been satisfied by a suitable switch structure.